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In navigation, a bearing is the angle between the direction to an object and a reference direction. Unless otherwise specified, the reference direction is generally understood to be magnetic North, in which case the term compass bearing is also used. If navigating by gyrocompass, the reference direction is true north. In stellar navigation, the reference direction is that of the North Star, Polaris.
Generalising this to two angular dimensions, a bearing is the combination of antenna azimuth and elevation required to point (aim) an antenna at a spacecraft. The bearing for geostationary satellites is constant. The bearing for polar-orbiting satellites varies continuously.
Moving from A to B along a great circle can be considered as always going in the same direction (the direction of B), but not in the sense of keeping the same bearing, which applies when following a rhumb line.
Accordingly, the direction at A of B, expressed as a bearing, is not in general the opposite of the direction at B of A.
For example, A and B on the northern hemisphere have the same latitude, and at A the direction to B is eastnortheast. Then going from A to B, one arrives at B with the direction eastsoutheast, and conversely, the direction at B of A is westnorthwest.
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